1. Field of the Invention
This invention relates to a heat-sealing heater for use in an overwrapper which applies heat under pressure to a wrapping film wrapped around an article such as a magnetic tape cassette and heat-seals overlapping portions of the film.
2. Description of the Prior Art
Conventionally, the heat-sealing heater for an overwrapper comprises a metal block such as of aluminum having a tubular heater element and a temperature sensor embedded therein, and the overwrapper applies heat under pressure to a wrapping film by use of the metal block and heat-seals overlapping portions of the film. Particularly, the conventional overwrappers for wrapping audiotape cassettes or videotape cassettes are arranged to owe the pressure required for the heat seal to repulsion of the article to be wrapped. Accordingly, depending on the shape of the portion of the article at which the overlapping portions of the wrapping film are heat-sealed, it is difficult to obtain a uniform heatsealing pressure. For example, the plastic casing for the audiotape cassette generally has recesses in the outer surface thereof which are about 0.5 mm in depth and are formed due to sinkage in injection molding of the casing. At the portions opposed to the recesses, the overlapping portions of the wrapping film cannot be pressed against each other under a sufficient pressure, which can lead to unsatisfactory heat sealing and to deterioration of the wrapping yield. Further, in the case of the videotape cassette, the rigidity of the material forming the casing is insufficient and accordingly, if the cassette accommodated in the casing has recesses, the overlapping portions of the wrapping film cannot be pressed against each other under a sufficient pressure at the portions opposed to the recesses.
Recently, there has been put into practice a flexible flat heater comprising a long heater element such as a winding embedded in a flexible flat material. Such a flexible flat heater may be expected to conform to some extent to the unevenness on the surface of the article to be wrapped and permit the overlapping portions of the wrapping film to be uniformly heat-sealed.
However, since the resistor circuit of the flat heater is generally formed by photo-etching a metal film about 10 to 100.mu.m thick in order to ensure flexibility of the heater, the flat heater cannot have a large heating capacity.
Also a flat heater having a Nichrome wire winding instead of the resistor circuit formed by photo etching of the metal film cannot have a large heating capacity.
Accordingly, when the flat heater is used as the heat-sealing heater for the overwrapper for wrapping the audiotape cassettes or the videotape cassettes, the temperature of the heater can be locally abruptly lowered at the moment when the heater is brought into contact with the wrapping film because the heat load of the cassette significantly varies from part to part. Accordingly, in order to obtain satisfactory heat sealing over the entire heat sealing area with such a flat heater, the heater must be pressed against the wrapping film for a longer time, which makes it difficult to increase the production speed. Though these problems may be overcome by the use of a rapid-response temperature control system, this approach adds much to the production cost.